The present invention relates to a laminated thermoplastic film having excellent gas barrier properties. More particularly, it relates to a laminated plastic film which has a high gas barrier property against steam and oxygen as well as transparency and is suitable for a variety of packaging materials.
Plastic films and their molded pieces used for packaging materials of foods, pharmaceuticals, chemicals and the like are made of gas barrier materials which are impermeable to gases such as steam, oxygen or the like in order to prevent the change of properties of a packaged content. In packages which require a high level of impermeability to gases, materials which have been additionally laminated with a metal foil such as aluminium or the like or on which metal such as aluminum or the like has been deposited are used.
Films made of a polyvinylidene chloride or a vinylidene chloride type resin such as a copolymer of vinylidene chloride as a main component and other monomers copolymerizable with vinylidene chloride such as vinyl chloride, methyl acrylate, methyl methacrylate, acrylonitrile or the like or vinylidene chloride type resin coated films in which films made of polypropylene, polyester, polyamide or the like have been coated with the vinylidene chloride type resin are also known as packaging materials having gas barrier properties.
Furthermore, a polyvinyl alcohol type resin such as polyvinyl alcohol or an ethylene-vinyl alcohol copolymer is also a material excellent in oxygen barrier properties. However, the polyvinyl alcohol type resin is inferior in steam barrier properties, and oxygen barrier properties are also deteriorated under a high humidity. Thus, when it is intended to use as a packaging material, it is usually formed into a laminated film that a steam barrier resin film made of polypropylene, polyethylene, polyester or the like has been laminated on the polyvinyl alcohol type resin film.
It has been proposed to afford gas barrier properties to a plastic film having an excellent mechanical strength by depositing a metal oxide film. There are, for example, films that silicon oxide has been deposited on a biaxially oriented nylon film or a biaxially oriented polyester film (Japanese Patent Publication No. 12953/1978) or that magnesium oxide has been deposited on a polyethylene terephthalate film or a biaxially oriented polypropylene film (Unexamined Published Japanese Patent Application No. 27532/1985).
Conventional packaging materials which use a metal foil or a metal deposition film are excellent in gas barrier properties to steam or oxygen, but these materials are always opaque because of the lamination of the metal. Accordingly, contents cannot be observed from the outside of the packaging material, and their use as packaging materials is limited within very narrow region.
On the other hand, conventional vinylidene chloride type resin films and polyvinyl alcohol type resin films can be formed transparent, but these films are not satisfactory in gas barrier properties to steam and oxygen. When these films are used for packaging materials which require a high level of gas barrier properties, the thickness of the films must be increased. However, if the thickness of the films is increased, the transparency or softness will be impaired. Accordingly, these films are inadequate to packaging materials which require a high level of gas barrier properties.
Further, a packaging material that a metal oxide film is deposited on a plastic film to afford gas barrier properties is also insufficient for gas barrier properties. Accordingly, it is unsatisfactory for the use which requires a high level of gas barrier properties.